Anatomy, Ecology and Ontogeny of Mesosaurs using 3D-Imaging Technologies

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Mesosaurs are a unique and very specialized vertebrate group, representing the first fully aquatic amniotes. Mesosaurids are commonly considered to belong to Parareptilia, although some studies suggest that they represent basal sauropsids outside of the split between parareptiles and eureptiles. They are exclusively known from the early Permian of southern Gondwana. However, despite their restricted stratigraphic and geographic range, they represent an important step in the evolutionary history of tetrapods: being highly specialized for an aquatic lifestyle by having a long skull with an extremely elongate snout, spiked with long and thin conical teeth, a slender neck, a long flattened tail for laterally undulating movements, and paddle-like fore and hind limbs. Only three monotypic mesosaurid genera are currently recognized as valid: Mesosaurus tenuidens, Stereosternum tumidum, and Brazilosaurus sanpauloensis. Although they represent a key group in the early evolutionary history of amniotes, mesosaurids are quite poorly known, despite some recent research advances on M. tenuidens. The main objectives of the proposed project comprise a detailed anatomical investigation of the remaining mesosaurids Stereosternum and Brazilosaurus using modern 3D imaging technologies from CT data to reveal new phylogenetic characters to test existing hypotheses about their inter- and intrarelationships. The planned 3D reconstruction of the general mesosaurid skull anatomy aims to resolve the controversial and historical question of whether the skull condition of mesosaurids is anapsid or synapsid. The proposed 3D analysis of the skull (including its internal cranial structures such as the braincase, cranial endocast and inner ear) aims to further fill a gap in our knowledge, and yield important data to understand the early evolution of the amniote skull. Moreover, a Finite Element Analysis (FEA) will be applied to skulls of Stereosternum and Brazilosaurus and compared to other extant and fossil secondarily aquatic tetrapods to test their respective bite power and skull resistance and to test whether there are differences in feeding behaviour (filter-feeders vs. active predators) and locomotion among the mesosaurid taxa. Finally, a thorough ontogenetic study with special focus on overall postcranial (but also cranial) proportions, applying a morphometric approach including linear measurements as well as geometric morphometrics aims to investigate potential variations in the growth patterns of mesosaurids, and hence whether there was a difference in feeding behaviour between juvenile and adult mesosaurids. The results of this research promise novel insights into the palaeobiology of these earliest secondarily aquatic amniotes.